This application claims the priority of Japanese Patent Application No. 4-280791 filed on Sep. 25, 1992 which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a signal processing circuit for an electronic endoscope which processes a video signal obtained by a solid-state image sensor so as to display an image.
2. Description of the Related Art
Electronic endoscopes used in medical and industrial fields are known. Such an electronic endoscope is inserted into a body cavity such as an alimentary canal and a structural body through an electronic endoscope provided with a CCD (Charge Coupled Device) as a solid-state image sensor at the end portion thereof so as to observe the body cavity.
FIG. 13 shows the structure of a conventional electronic endoscope. An electronic endoscope 1 is removably connected to a processor apparatus 2 through a connector. In the end portion of the electronic endoscope 1 are provided a CCD 4 which is optically connected to an observation window 3, a CCD driving circuit 5, a light projection window 6 for projecting observation light therethrough into a body cavity and a light guide (optical fiber) 7 for providing light to the light projection window 6. In the processor apparatus 2, a correlated double sampling (CDS) circuit 9 is provided so as to be connected to the CCD 4 through an amplifier 8. A sampling pulse generator 10 provides a sampling pulse for the CDS circuit 9. A video signal processing portion 11 for executing gamma correction, white balancing, etc. and having a memory or the like is connected to the CDS circuit 9. The processor apparatus 2 is further provided with a light source device 12 and a condenser lens 13. The light source device 12 provides predetermined light (RGB beams in a surface sequential processing system) for the light guide 7 in the electronic endoscope 1.
According to the above-described structure, the light output from the light source device 12 is projected into a body cavity through the light projection window 6 of the electronic endoscope 1, so that the image of the object of observation is taken by the CCD 4 which is driven by the CCD driving circuit 5. A video signal output from the CCD 4 is supplied to the CDS circuit 9 through the amplifier 8. The CDS circuit 9 subjects the output of the CCD 4 to clamping processing, and samples and holds the video signal portion which contains image information. In this case, the sampling processing is executed on the basis of the output of the sampling pulse generator 10, and the video signal portion is sampled and held by a sampling pulse whose phase is adjusted in relation to the clamping pulse. Due to the correlated double sampling, the noise in the video signal is favorably reduced.
After the gamma correction processing of the output of the CDS circuit 9 by the video signal processing portion 11, the signal is output to a monitor which is connected to the processor apparatus 2. In this manner, the monitor displays the color image of the body cavity as the object of observation.
However, when the conventional signal processing circuit for an electronic endoscope is used for electronic endoscopes 1 having different lengths, the timing of clamping, sampling, and holding the video signal output from the CCD 4 varies in accordance with the length, so that it is impossible to effectively operate the CDS circuit 9. There are electronic endoscopes 1 having different lengths in correspondence with the objects of observation. If electronic endoscopes 1 having different lengths are connected to the processor apparatus 2, it is necessary to provide the sampling pulse and the like for the CDS circuit 9 with an operation timing which is determined in consideration of the time for transmitting the video signal through the length of the corresponding electronic endoscopes 1. In this case, the processor apparatus 2 may be provided with a circuit or the like for changing the operation timing depending on the length of the electronic endoscope 1. Then the structure of the circuit becomes complicated, and it is laborious to exchange the electronic endoscopes 1.
In order to solve this problem, a sample-and-hold circuit is conventionally provided at the end portion of the electronic endoscope 1 on the same chip as the CCD 4 or independently thereof, as disclosed in Japanese Patent Publication No. 75118/1991. In this case, however, the outer diameter of the end portion becomes large, which is an obstacle to the reduction in the diameter of the electronic endoscope. In addition, when the sample-and-hold circuit is provided at the end portion, the consumption of electric power at the end portion becomes large, which results in the rise in the temperature of the end portion which is to be inserted into a body cavity and an increase in the nonuniformity of dark current which is generated on the surface of the silicon substrate of the CCD 4, thereby deteriorating the performance of the CCD 4.